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Mediterranean
Bluefin Tuna:
An ancient fishery copes with modern demands
John
Mark Dean1, Piero Addis2 and Angelo Cau2
SUMMARY
For more than 2,600 years,
the bluefin tuna (Thunnus thynnus,
Linnaeus, 1758), has been a renewable
natural resource and sustainable fishery in the Mediterranean Sea. This great fish has provided the economic base for many
Mediterranean coastal communities for centuries. The accumulated scientific evidence shows that modern
development activities and current fishing practices threaten the bluefin tuna
population of the eastern Atlantic Ocean and the Mediterranean. We analyzed the annual catch data from the years of 1825-2000
of archival and modern catch records to reconstruct the modern history of this
ancient fishery. Those analyses reveal significant declines in abundance have
occured over time (mean, 1825-1911 =7,572/yr, 1912-1980= 2,990/yr). Important
biological parameters (length, weight, sex, and ovarian development) were
measured directly and by observation from collections made at the traps for the
years 1992-2000. The sex ratio was calculated on a sampling of >3000
individuals from all size classes for each year. We applied length data and ICCAT parameters, from the catches
during that period, to estimate the age of individual fish.
Changes in size classes, sex ratios, reproduction and current fishery
practices are considered. We
discuss the impact of the contemporary fishing method of purse-seine fishing on
the future of the Mediterranean bluefin tuna resource and conclude the
traditional trap fishery is an ecologically compatible gear.
1, Belle W. Baruch Institute
for Marine Biology and Coastal Research, University of South Carolina, Columbia,
SC 29208 USA
2. Department of Animal
Biology and Ecology, University of Cagliari, 09126 Italy
Running
head: Mediterranean Bluefin Tuna and fishing methods
INTRODUCTION
The bluefin tuna (BFT), (Thunnus
thynnus, Linnaeus, 1758), is
documented as an important part of the culture of the Mediterranean for 12,000
years and has been a sustainable
fishery in the Mediterranean Sea for at least 2,600 years, and probably much,
much more. Over that long period of
history, and to this day, the magnificent bluefin tuna have been a source of
food, and income and a central core of the culture for the people of the region.
The fish provides an economic base as a source of employment and income for
fishermen, net and boat builders, processors, and sellers of tuna products for
coastal communities of the Mediterranean Sea, and the Atlantic coast of the
Iberian Peninsula and north Africa. In the past, bluefin were called “La manna
del Mediterraneo” (Tyndale, 1849).
For extended periods of time,
this highly migratory fish ranges over the Mediterranean Sea and in the Atlantic
Ocean, from 60°N
Lat. to 40° S
Lat, feeding on squids, pelagic crabs and fishes. Their extensive oceanic
migration is followed by an aggregation of schools that enter the Strait of
Gibraltar, and distribute themselves to their ancestral spawning grounds in the
Mediterranean Sea. Resident
Mediterranean BFT also migrate to form aggregations on ancestral spawning
grounds. The distribution of
bluefin larvae shows that spawning occurs throughout the Mediterranean Sea (Ueyanagi
et al, 1997, Tsuji et al, 1997, Piccinetti et al,
1997, Nishida et al, 1998, Oray and
Karakulak, 1999). Spawning sites
appear to be concentrated in the area of the Balearic Islands, the Tyrrhenian
Sea southeast of Sardinia, with some spawning occurring in the Aegean Sea and
the southern Mediterranean coast of Turkey (Oray and Karakulak, 1999).
Aristotle (Tyndale, 1849, and by D. Thompson in Smith and Ross, 1910)
considered the Black Sea to be the most important spawning area for
Mediterranean BFT, but they no longer pass through the Dardanelles, Sea of
Marmara or the Bosporous to spawn in the Black Sea (Bok, 1991, Oray and
Karakulak, 1997). The great tuna
fishery of the Black Sea and Bosporous, that created immense wealth for the
city-states of Kyzikos and Melitus of ancient Greece, exists no more.
Adult bluefin have been
captured for thousands of years in a trap system (Fig. 1) that is suggested to
have been introduced to the western Mediterranean basin during the emigration of
the Arabs in the 7th Century AD (Smith, 1968).
However, when the Arabs first occupied Sicily they found a vigorous tuna
fishing industry in place. During
the period of Arab-Norman Sicily (800-1200 AD) and in medieval Sicily, fishing
rights specifically for tuna, were often granted as royal prerogatives (Smith,
1968). The trap technology, which
was usually located in close proximity to solar evaporation salt production
sites, was extremely common throughout the Mediterranean, Dardanelles, Sea of
Marmara, Bosporous and Black Sea. Cetti
(1777) in his classic 18th century natural history of Sardinia showed
the location of 35 traps on the western coast of Sardinia.
Large numbers of the fish are
produced from the June-July spawning period in the Mediterranean. Hundreds of
thousands of juvenile BFT emigrate through the Strait of Gibraltar to the
Atlantic Ocean in the fall, when they are about 30-45 cm fork length (2-4 kg) (Rey
and Cort, 1986). And, hundreds of
thousands of juvenile BFT are captured by Moroccan, Spanish and French fishermen
in the Atlantic Ocean and Bay of Biscay (Cort, 1990).
Spanish, French and Italian purse seiners and hand-line fishermen harvest
additional thousands of small BFT in the Gulf of Lyon (Farrugio, 1980), the
Ligurian Sea and the Adriatic Sea (Piccinetti, 1980). A very large percentage of
those BFT of are well below the legal minimum size, 6.4 kg and 69 cm fork length,
established by The International Commission for the Conservation of Atlantic
Tunas (ICCAT, 1975).
The accumulated scientific
evidence shows that modern development activities and current fishing practices
threaten the BFT population of the eastern Atlantic Ocean and the Mediterranean.
ICCAT estimates that the 1996 stock size of fish larger than 145.5 kg (fish
of estimated age classes nine or more) is only 23% of the 1970 stock size (ICCAT,
1997a). The ICCAT Executive Summary
(ICCAT, 2001) discusses the extensive catches of recent year and particularly
the increase in purse seine catches. The
Scientific Committee of Research and Statistics (SCRS) of ICCAT stated that a
catch of 25,000 MT is necessary to stop the decline, but landings are estimated
to routinely exceed 30,000 MT (the catch was 52,644 MT in 1996).
The bluefin is in trouble and, with its decline, many important cultural
elements of the Mediterranean Sea are also at risk and could be lost unless
precautionary measures are implemented. The
Mediterranean Sea is the cradle of western civilization and we should be
concerned about the integrity of something so basic to the economic and cultural
history of the region as this great fishery.
Ironically, even though this fish has such an important place in history,
and is currently a highly valued fish on the
international market, much of the fundamental biology and ecology of the bluefin
still remains unknown. We therefore conducted detailed analyses of the reproductive
biology of the fish captured in the traps.
In an effort to better
understand both the biology of BFT and its relationship to the cultural history
of the region, we conducted a critical analysis of historical landing records
from two tuna traps, Portoscuso and Isola Piana, in southwestern Sardinia for
the years 1825-2000 (Fig. 2). The trap at Portoscuso was established with a
charter from Philip II of Spain in 1496 and intermittent records from the
catches of the 16th - 18th century exist.
An uninterrupted record of historical annual catch data, from 1825 to the
current day, is available from the archival records of the Isola Piana and
Portoscuso traps.
Extensive documentation,
anecdotal observations from very experienced trap fishermen, market records, and
current data, show that there have been significant periodic declines in the
historical Sardinian trap landings (Cau et
al, 2000). It also appears that the peak catch period in the traps has
shifted from late to early in the season. We
have found that the reproductive biology of the tuna in this area differs
significantly from the assumptions used in current population assessments. We hypothesize that the two major declines in catches in the
traps are due to industrial development and new industrial fishing practices.
MATERIALS
AND METHODS
The traps used for analyses
in this study are located on the southeast coast of Sardinia at 39 degrees N
lat. and 8 degrees E long. within 10 km of each other (Fig.2). We collected data
from 3800 samples at the annual harvest of the Portoscuso and Isola Piana traps
during 1992-2000.
Additional data have been retrieved from the trap's private records
and historical archives (Addis et al,
1996). We analyzed the individual
and collective annual harvest of the traps by conducting time series and trend
statistical analyses. There is no statistically significant difference in the
catch records from the two traps, so the results have been merged for the
analyses. We needed to test for
possible differences in the daily catch distribution over the time frame of the
trap operation. We examined the
historical records for periods that had complete continuous data for daily
catches that were separated by at least 50 years.
The periods of 1829-1844 (N. of fish, 7,674), 1950 to 1971 (N. of fish,
3,909) and 1993-2000 (N. of fish, 8,100) met the criteria and were selected for
detailed analysis
A curved lower jaw fork
length (CFL) measurement, made with a metal tape measure, and round weight (RW),
made of individual fish on a commercial electronic scale, was made at the dock
when the fish were unloaded from the boat.
The sex and dressed weight (DW) of the individual fish was obtained in
the refrigerated processing room. Some observers that collect data on landings
of bluefin tuna have access only to dressed fish.
For this reason, it is very useful to have a similar measure of the fish
from the Sardinian traps. Dressed
weight refers to fish without the entrails, which we consider to be the gonads,
gastrointestinal system and heart. The
RW/DW relationship was obtained from 350 fish with RW and DW values of 27-405
Kg. Data which are important
components of population dynamics models (Clay and Hurlbut, 1985), include:
length/weight ratio (CFL/RW), size class, size frequency for the total catch for
the year, estimated age classes, ovarian development, sex ratio by year, and
overall sex ratio.
All fish captured in the trap
are fully sexually developed so the visual determination of the sex from the
gonads is readily done. Sex of individual fish, and stage of ovarian development,
was verified with light microscopy histological sections using standard methods
as described in Olla (2000). The
sex ratio (male/female) is based upon 3,312 observations from 1993-1999.
The size classes were converted to estimated year classes on the basis of
bluefin tuna growth parameters used by the Standing Committee on Research and
Statistics (SCRS) of The International Commission for the Conservation of
Atlantic Tunas (ICCAT) (1997a), to estimate the age frequency of the catch for
each year. We used a total of 1731
directly measured length and weight values (CFL, 87-300 cm) to determine the a
and b parameters of the equation W = a * kg; 472 values referred to the females
(CFL, 87-295 cm) and 562 values referred to males (CFL, 89-300 cm).
RESULTS
The historical data shows
that 878,000 BFT were caught in the traps of Portoscuso (482,000 tuna) and Isola
Piana (396,000 tuna) over the 155 year period from 1825 to 1980. The catches
ranged from 0 fish/yr (there was no fishing in 1867 and 1943) to a maximum of
18,500/yr (1910) (Fig. 3). Fishing stopped for a period of time from 1981-1990.
A dramatic decline in Sardinian trap catches was recorded at the
beginning of the 20th century as total annual landings fell from an average of
7,572 fish
per year prior to 1912 to 2,990 fish per year after 1912. The next significant
change in the catch of tuna from these traps occurred in the 1970s when catch
declined from about 3000 tuna per
year to less than 1,000 per year. From the late 1800s to the 1970s, the bluefin
were harvested, processed and preserved in salt and oil in barrels and for the
canned tuna market (Rubino, 1994). The
traps were taken from the water in the 1980s, as the catches were not sufficient
to support the canneries. Traps
were put back in the water in the early 1990s with a harvest dedicated to the
lucrative export market of Japan.
Historically, the trap
fishery in Sardinia was conducted from early May to mid-June. From a comparison
of the historical series, it can be seen that the number of fish caught and the
peak time of the trap harvest in the current fishing period is very different
than in the past. In the 1829-1844 period, 26% of the catches were in May and
74% were in June while in 1950-1971, 45% of the catch occurred in May and 55%
was in June. In 1993-2000, 79% of the fish are caught in May and
only 21% are taken in June (Fig. 4). Other
traps of Sardinia, specifically those in the Gulf of Asinara, in northern
Sardinia, had a more prolonged fishing season, which began in early May but
lasted to the end of July (Rubino, 1994). Currently,
tuna schools are so small and infrequent in June that the catch in the traps has
been very low. In addition, the fat
content of the tuna muscle is low in June that reduces their value in the
Japanese fresh auction market. In
2001, catches were made in the Isola Piana trap in April, which is earlier than
previously recorded (Addis, pers. Comm.).
The data obtained from the
tunas captured by the traps at Isola Piana and Portoscuso during the 1992-2000
period yielded the following results for the CFL/RW relationship (Fig. 5): WT =
4.43 * 10-5 CFL 2.8171
, r = 0.98. Males
and female data were combined so our results can be compared with the other data
sets that do not differentiate the sexes. Our results differ from the data in
the literature (Fig. 6) as Rodriguez-Roda (1964) obtained a value of WT=1.9*10-5FL3
while Srour (1993) found the equation was 1.8*10-5FL3.0096.
We also tested the variance in CFL/RW during the season and found no significant
difference in the CFL/RW ratio seasonally or with the sex of the fish.
We calculated the length (Fig.
7) and weight (Fig. 8) frequency of the catch for each year and we show the
weight frequency in 1993 vs 2000 for a detailed comparison (Fig. 9).
There are some obvious shifts from the early 1990’s when fewer but
larger fish were caught and more recently, when more fish are captured but the
average and modal size has decreased. In
addition, by using the ICCAT conversion equations and applying them to the same
data, it is possible to estimate the age composition of the fish landed at the
trap (Fig. 10). Again we see a
reduction in the size and estimated age of fish during the recent time frame. The dressed weight/round weight relationship was DW= 0.8555
WT - 1.8999, r = 0.9818 (Fig. 11). The
dressed weight values range from 12 to 33% with a mean of 20% of the total
weight for the tunas from the traps.
Visual examination of the
ovaries of more than 100 small (20 kg) fish captured during the current study
period showed unequivocal evidence of complete oocyte development.
Histological preparations of ovarian tissue were made on a wide range of
size classes for confirmation. Olla
(2000), in a preliminary investigation, found that small BFT (105-110 cm of ljfl
= 20 kg) from the Sardinian traps are capable of reproduction. Those fish belong
to the presumed 3rd year class with a large number of clearly
identifiable oocytes in the immediate pre-spawning 3Y stage, (Yamamoto, 1956 and
Forberg, 1982) (Figs. 12,13). The
3Y oocytes occupied 60% of the total volume of the ovary. That data supports the
macroscopic observations that small females are reproductively capable.
Ovarian development progressed in the May-June period in all size classes
of females (Fig. 14). Fecundity
estimates were consistent with literature values.
An important analysis of any fish population is knowledge of its sex
ratio. The sex ratio in the fish
captured in the Sardinian traps was slightly above 1.0 for the last 7 years with
a mean of 1.15 (Fig15).
DISCUSSION
The SCRS reports that the
bluefin tuna that spawn in the Mediterranean Sea are harvested beyond the
capacity of the fish to recover to even pre-1970 population levels (ICCAT,
1997b, 2001). The SCRS reports that there has been a strong decline in the
abundance of older and/or larger fish since 1993 that corresponds with a large
increase in fishing mortality (ICCAT, 2001).
The mortality during that period is primarily due to long-line fishing,
drift gill nets and increasing purse seining efforts on large fish.
There is some indication of increased recruitment of small fish but
fishing mortality has increased on all size classes.
Model projections indicate that the level of catch, (1996=52,644 MT,
1997= 49,493 MT, 1998= 38,418 MT and 1999 = 30,868 MT, ICCAT, 2000a), is not
sustainable and a catch level of less than 25,000 MT is necessary to stop the
decline in biomass (Cort, J. Personal communication, 2000; ICCAT, 2000b).
ICCAT estimates that the present size of the stock is declining at a
significant rate and that the current numbers are only 20% of the level of 20
years ago. ICCAT’s SCRS has
recommended that the level of fishing effort by all countries that harvest the
presumed eastern bluefin stock be reduced.
Each country that uses the eastern BFT resource was given an allocation
of total allowable catch in 2000 and 2001 (ICCAT,
2000c). Several countries filed an objection under ICCAT procedures and
proceeded to significantly overfish their allocation. Compliance is clearly very difficult to achieve as fishermen
are not willing to reduce their capture of bluefin, nor do governments comply by
implementing and enforcing ICCAT recommendations.
The analysis of the
historical data, together with some events related to the socio-economic
development of the areas involved with tuna fishing with the trap net system,
has led us to the following possible explanations for the observed decreases in
the catches. The catches in these
two Sardinian traps ranged from 18,600 to 3,200 fish per year in the period of
1860 to 1911 (Fig. 3). There were
only five years during that 52 year span when the catch was less than 5,000 fish/yr
while in 24 of the years, there were more than 10,000 fish caught in the two
traps with an average of 9,137 fish/yr. The
last year the catch exceeded 10,000/yr. was 1911 and it has never reached that
level since.
Fishing effort, which is the number
of days that the trap net is in place and actively fishing, remained the same
during all that time and to the present day.
The catches declined in the early years of the period from 1912-1971 from
a high of 8,018 to a low of 894 fish/yr with an average of 3764 fish/year.
There are two possibilities
to explain this change in catch. One
explanation is that it is the result of natural variation in the population and
the second is that the decline coincides with the development of mining areas in
southwestern Sardinia (Manconi, 1986). Lead,
zinc and copper ores was mined in drainage basins of the region in the late 19th
and early 20th centuries. The
traps are in the watershed of the production areas. Waste generated in the
processing of the ore consisted of fine sediments and heavy metal residues
entered the river draining into the sea in the immediate vicinity of the Porto
Paglia and Portscuso traps. The assumption was that those sediments caused a
serious decline in the catch of tuna in those traps. The trap license holder brought a lawsuit against the mining
company on the basis that the mines had a significant negative effect on the
water quality of the area that reduced the catch of BFT.
The court found in favor of the plaintiff and the court awarded damages
to the license holders of the traps.
We
tested the hypothesis that the reduction in catch was correlated with the mining
development by comparing the annual catch records of the Porto Paglia and Portoscuso traps with the
records from the La Tonnara Saline trap (Fig. 2), which is located on the
northern end of Sardinia. The
hypothesis is that the catch in La Tonnara Saline would not decline as it is far
removed from the effects of the mine effluents and prevailing currents.
There was no difference in the catch trends of the three traps as the
catches also declined at La Saline during the same years (Rubino, 1996).
Thus we are left with the distinct possibility that a decline occurred in
the bluefin population that presumably could have been reflected throughout the
Mediterranean (Fromentin
et al. 2000).
The catch was 2,940 fish in
1972 but catches declined substantially throughout the 1970’s until the catch
was so low that it was not economically viable to fish the traps and fishing
stopped in both traps in 1979-80 and there are several possibilities that might
explain those reduced catches. The
first is that a major industrial area for aluminum production developed near the
main traps of southern Sardinia. Large
amounts of red waste sediment residues from the plant and stored in a retention
basin washed into the sea in 1974 when the retaining wall failed. There were no catches were made in the nearby Portoscuso trap
that year with reduced catches for several years thereafter.
Ship traffic also increased during that time to support the industrial
development causing major changes in underwater sound as well as water quality.
It should be recognized that such changes in the Mediterranean coastline
for industrial and tourism development is not limited to Italy but has occurred
throughout the region. The highest catches in the historical record were
consistently recorded in the now highly impacted Portoscuso trap.
The trap at Isola
Piana, near Carloforte, is located in the waters that are the least subject to
ship traffic and runoff from the land or other sources of pollution.
The IP trap currently has the highest catches of bluefin.
A
major factor that might be responsible for reduced catches in the introduction
of new types of fishing gear, specifically the introduction of longlines, drift
gill nets and purse seine boats with spotter aircraft. Because
of their effectiveness, spotter aircraft have now been removed from the fishery.
Longlines and purse seine methods are relatively recent as they have only
been fully developed in the last 30 years and utilize different technological
advancements. Those fishing methods were vigorously developed and exploited in
the late 1960s and the Japanese, French, Italian and Spanish fishing industries
developed purse seine technology to a high state in the early 1970s. The
reduction in catch in the traps in the Mediterranean correlates with the
introduction of the new types of fishing gear.
There is also a small but
significant, and increasing take of BFT by the recreational fishing industry.
Recreational fishing is effective in those regions where the larger bluefin are
past the spawning period and are actively feeding again.
It can also occur where there are significant concentrations of juvenile
bluefin. However, there is little
discrete recreational catch data available.
Since recreational landing statistics are very difficult to collect, the
catch probably exceeds the quota. The
very positive economic benefit from recreational fishing can be large in
relationship to its possible negative impact on the bluefin population.
The negative impact of a recreational harvest can be significantly
reduced by the practice of capture and release, a behavior of recreational
anglers that has become very prevalent in the United States recreational fishing
industry.
The most compelling evidence
that the most recent decline is the result of the increased activity of purse
seine fishing is that the fishery concentrates on the capture of small tuna of
less than reproductive size (Fig. 16). It
is estimated that traps along the coastline of the Mediterranean take less than
10% of the BFT captured in the Mediterranean each year whereas longline fishing
and purse seines, take most of the bluefin.
A significant commercial handline fishery that targets small bluefin
exists in virtually all the states bordering the Mediterranean and in the Bay of
Biscay.
Beginning in 1990, ICCAT
required the reporting of landings of small fish, those less than the ICCAT
minimum size regulation of 6.4 kg, with an allowance of 15% of that size
category of BFT as a portion of a country’s annual landings.
The record shows that a very large number of small fish continue to be
harvested each year (Fig. 17) and they far exceed the allowance regulation. (ICCAT,
1997a). In addition, most ICCAT
nations do not even report their undersize catches, as required by ICCAT (ICCAT,
2001). According to ICCAT
statistical data, the first records of the decrease in total eastern Atlantic
and Mediterranean bluefin catches were made in the late 1960s. Trap catches
began a sustained decline in total landings and a significant decline in their
percentage of the total catch after the onset of purse seining (Fig. 16). During that time,
there was a progressive increase in purse seine boat tonnage and their catches,
whether expressed as total landings or percentage of the total catch.
Most importantly, they landed significant numbers of less than legal
minimum size fish (Fig. 17). To a
lesser extent, the increased catch is also mirrored in longline catches.
Thus, for the first time in
thousands of years, mature bluefin are harvested as large fish by traps,
longlines and selective purse seines, while at the same time, juveniles are
landed in huge numbers by purse seine and handline fishermen.
A result of the intensive harvest of all size classes is the change in
the size or age structure of the catch. There
has been a significant reduction in the average and maximum size of the bluefin
caught each year (Fig. 9, 10). Data
from ICCAT show a significant reduction in the number of big, and presumably
older tuna, over the last nine years (Fig. 9).
Similar results are obtained from the captures in the traps (Fig. 9).
There are a very limited
number of directly determined estimates of the size at age of very large
Atlantic bluefin tuna, but a bluefin tuna with a CFL of 267 cm (converted weight
= 300 kg) was estimated by Kalish (1998) to be more than 33 years old.
It is important to recognize that large old fish are the reservoirs of
the specie’s historical genotypes and keeping those fish in the population
pool is necessary for the maintenance of genetic diversity. The most important way to maintain genetic diversity is to
have the largest possible number of individuals contributing to the population.
That is a difficult argument to make in an aggressive and highly
profitable fishery.
Another feature that the data
show is the shift in the peak period of catch in the tuna trap fishing season,
and it has been reduced in modern times (Fig. 4).
Historically, the season lasted from mid-May until early July, with the
majority of the catches occurring in June.
Now, the majority of the catch occurs in May and the season is over by
early June. This could be explained
by a change in the migratory routes of the stock over time, fishing practices
with other gear, coastal development, the development of Atlantic Ocean
fisheries, climate shifts and the physical processes in the sea.
It is also possible that the recent harvest, which has significantly
reduced the numbers of very large fish and resulted in a decrease in the average
size of reproducing individual fish, has shifted the spawning season for the
remaining tuna. The local fishermen
think that purse seine fishing breaks up the tuna schools, from a large
homogeneous school that seems to behave as a single unit, to smaller schools
that do not behave in such an organized manner and they are more vulnerable to
external factors. A final
possibility that we have considered is that there has been such a significant
decrease in the absolute number of fish in the population so it simply takes
less time for the migrating bluefin to pass the trap locations. These are
reasonable ideas with some supporting evidence, but all are extremely difficult
to frame as testable hypotheses. Presently,
there is not enough data to support any one explanation over any other and
further research is necessary to cope with these important questions.
The length/weight
relationship for Sardinian trap fish differs from that determined by Srour
(1993) for Moroccan traps and from that used in ICCAT reports (ICCAT,
1997c). The results we
obtained are closer to those calculated by Rodriguez-Roda (1964) from the western Mediterranean (Fig. 6).
This could be the result of several factors. Our data is obtained from a
group of fish that are collected in a relatively small area.
The Portoscuso and Isola Piana traps are within 10.0 km of one another,
and the site is in the middle of the Mediterranean.
In addition, the fish collected in the traps are landed in a short period
of time. Differences in the
estimation of this size relationship are important because of the impact it can
have on the assignment of the fish to different estimated age classes that are
used to monitor annual recruitment, calculate mortality and conduct stock
assessments.
There is a significant
increase in the development of the gonads during this roughly 30-day period when
the trap fishery operates. The
development is primarily a shift of fat from the muscle tissue to the gonads.
All stomachs are collected, opened, cleaned and processed as a food
product. There is no evidence that
the tuna are feeding during this period, as all the stomachs are completely
empty and the fish do not regurgitate food during the trap harvest.
In addition, attempts by recreational anglers to catch the fish during
this time are dramatically unsuccessful, as the fish do not respond to lures or
bait during the pre-spawning aggregation. The
bluefin are not physiologically close to utilizing tissue protein as a source of
energy either. It is clear that
there is a decline in muscle fat during this period, as each fish is evaluated
for the market and muscle fat is the single most important factor in
establishing the market value of the individual fish.
The gonad weight is estimated to be about 80% water. Therefore, an
increase in estimated gonad weight of from one to three kg (which is common in
mature >100 kg fish) during the 30 day fishing period would represent a
change which is less than the weight differences due to some dehydration during
transport to the dock and scale accuracy.
The fish in the industrial
and recreational fishery of the Mediterranean in the late fall and winter could
show shifts in CFL/RW ratios and condition factors over time.
The tuna must replenish the fat used for their spawning migration and
their conversion of muscle fat to ovarian development. They must capture the fat, energy rich prey that contain the
necessary calories for the bluefin to achieve their annual 15-20 kg increase in
muscle and organ body mass. The
companies that buy BFT for the export market are more knowledgeable about such
changes than the scientists.
In the survey by de La Serna et al. (1997) of the Barbate trap during 1989-1995, it is
conspicuous that the majority of the tuna captured by the Spanish trap during
their harvest season are females. Only
in 1994 did the ratio fall below 1.0. There were a significantly higher number
of females in the Barbate population in the smaller size classes than we have
measured in Sardinia. Our data show
a ratio very close to 1:1 throughout the length or weight range except in
extremely large fish. The sex ratio
in the Sardinian traps in the 1992-99 period ranged from 0.88-1.29 with an
average of 1.15. It is important to
recognize that the Barbate trap that de La Serna et. al. sampled is in the
Atlantic Ocean and north of the Strait of Gibraltar.
It is therefore directly influenced by the oceanic migratory nature of
the bluefin. The difference in sex
ratio and the size classes between the trap in the Atlantic and those in the
Mediterranean is particularly interesting.
The Atlantic Ocean fish are in the later stages of their spawning
migration, but have not yet reached the spawning grounds and are not yet in a
well-developed spawning state. In
the Mediterranean, fish are clearly approaching spawning condition (Fig. 12, 13,
14) and are very close to their spawning grounds. It is possible that sexually mature males and females can
occupy different areas of the ocean throughout the year but converge on the
spawning grounds at spawning time. It is conceivable that there is a difference
in the arrival times of males and females as they approach the spawning grounds.
The males could arrive earlier than the females and thus skew the catch data.
A significant finding in this
study is the fact that female BFT of greater than 250 cm (calculated from the
ICCAT conversion equations to be 322 kg), are routinely collected in the trap.
The ICCAT summary document (ICCAT, 1997a, Table 4) states that the sex ratio is
1/1 up to 230-240 cm, and then decreases to 0% females at about 285 cm.
We do not find this to be true for the fish we have sampled.
We have no knowledge of the age structure of the fish in these size
ranges.
Fishing gears have different
effects upon fisheries. Some gear
is more selective for its target species than others and controversy on gear
selectivity is a major issue in fishery management.
All bluefin currently captured in the Sardinian traps in the
Mediterranean are capable of spawning. That
fact is also true of the bluefin captured in the purse seine fisheries of Greece
and Turkey (Megalafonou and Karakulak, pers.
comm). We have regularly observed female tuna of 20 kg with well-developed
ovaries and a few precocious males of 15 kg that produced milt.
All males and females, and significantly those in the presumed year three
size class, that we have observed during the 1992-2000 trap harvests are clearly
sexually capable. This is not to argue that the entire population in the
Mediterranean has this characteristic, although the BFT from Tunisia and Turkey
do. There is the remote possibility
that sampling with other gear and other sites could possibly yield different
results.
ICCAT fishery population
scientists assume that only BFT of an estimated age five year class (136cm or
50kg) are 100% sexually mature (Table 4 in ICCAT, 1997b).
That assumption is the basis upon which the data is entered for the stock
assessment calculations. According to the ICCAT length to estimated age
conversion table, a 20-kg fish is three years old.
ICCAT reported (1997a) that 297,325 three and four year old fish were
landed in 1995. According to our
landings records, a significant number of small reproductively capable fish (20
kg, year three size class) have not been included in the data used by ICCAT for
the population estimates. In
addition, there are large females (although they are few in number) that are not
considered in the assessment models. Larger
females produce more eggs, and the assumption that all large fish are males
could alter the calculations on reproductive output.
These results emphasize the need for a complete review of the
reproductive biological parameters of bluefin tuna in the Mediterranean Sea.
It is quite possible that the current assumptions used by ICCAT have
serious limitations because they are based upon too limited a design for
sampling and analyses.
Survival from birth to
maturity and age at maturity are of primary importance to population growth and
viability (Hutchings, 2000). Is the
size and age of reproduction that we have measured, which is significantly lower
than that used in the stock assessments, an adaptive response to the modern
increased mortality rate as a result of enhanced fishing effort (Purdom, 1979,
Stearns and Crandall, 1984, Roff, 1983 and Nelson and Soulé,1986).
Such a phenomenon has been extremely well demonstrated in plaice, Pleuronectes
platessa (Rijnsdorp, 1993) and several other species of teleosts.
The fisheries literature has
numerous well documented examples of fishes that are very vulnerable to
overfishing if they have a late age of maturation, but were harvested at a young
age, prior to maturation (Myers and Mertz, 1998.)
It has been unequivocally demonstrated in most fishes that harvesting a
constant quota from a declining stock will cause an increase in selection of
smaller and presumably younger fish. According
to Myers and Mertz and others, “fish should be permitted to spawn at least
once before they become vulnerable to commercial gear”, and, we emphasize
recreational harvest as well. The
best safety margin is when the difference between the age at maturity and age at
which the fish are first captured in the fishery is greatest.
Specifically, a minimum size limit of 30 kg would insure that 100% of the
Mediterranean bluefin that spawn in the sea around Sardinia, and those in the
eastern basin of the Mediterranean Sea (Karakulak, Pers. Comm.) would spawn at
least one time before they were harvested.
Such a management measure would be the best way to insure the sustained
harvest of bluefin tuna and would constitute a valid part of a recovery program.
It must be recognized that such a recommendation could result in the
possible elimination of the Bay of Biscay bluefin fishery that concentrates on
small fish, would have extremely serious social and economic impacts.
As a result, the domestic and international political opposition to such
an action would be most severe.
A rational method to protect
juvenile bluefin in the Mediterranean, would use a biologically based size limit,
and both seasonal and area closures. The
evidence shows the targeting of small bluefin by purse seiners in the western
basin of the Mediterranean is a major source of the problem for the BFT
population. Many nations have
excessive fishing capacity and do not practice a precautionary approach as is
called for by FAO (1995). ICCAT has
recommended that landings be reduced in the eastern Atlantic Ocean and the
Mediterranean Sea and that other ICCAT recommendations should be fully
implemented.
The total allowable catch recommended by ICCAT for 1999 was 32,000
metric tons and 29,500 metric tons for the year 2000 (ICCAT, 2001). However, Morocco and Libya have said they will catch an
additional 4,598 MT and non-Contracting parties, Entities and Fishing Entities
will catch 2,291 MT for a minimum landings of 36,389 MT.
This catch level is significantly higher (46%) than the level the SCRS
Committee advised (25,000 MT) is necessary to achieve a sustainable stock.
The most fundamental problem
for the BFT is that there is an excessive amount of capacity to capture fish in
all the countries surrounding the Mediterranean. The purse seine boats in the
western basin of the Mediterranean and in the Atlantic Ocean are not selective
on the size of the fish in the school and regularly take large numbers of
bluefin tuna of less than the legal minimum size (ICCAT, 1997a).
Purse seine fishing takes place in all months of the year in the
Mediterranean, unless prohibited in specific areas by international agreements
under ICCAT or by national regulations.
We think that the tuna trap
is an excellent example of an ecologically compatible fishery technology.
The fundamental facts are that the traditional trap that has been used
for probably more than a thousand years yielded a sustainable fishery for the
following reasons as it:
§
Selectively and passively
harvests only sexually mature individual tuna.
§
Operates for a very limited
time of the year and the result is a very low fishing effort as compared with
other fishing technologies.
§
Depends upon environmental
cues for the concentration of the tuna.
§
Is a passive technology that
depends upon swimming behavior of the tuna.
§
Does not have significant
by-catch.
§
Does not
pursue the tuna beyond their local area of concentration.
§
Is
possible to release fish with a high probability of survival and can be used to
tag and release tuna for scientific research.
§
Can
capture and hold tuna to preserve the highest quality for the market.
§
Can
hold fish in the trap for several days so they can shipped when the market is
the best.
We suggest the critical
issues in the management of bluefin tuna are not 1) the development of new
fishery regulations for tuna management, 2) a need for more research on the
fundamental biology of the fish, or 3) the development of new models of
population dynamics.
Although those are desirable activities, at present many of the
recommendations from ICCAT's own SCRS, as well as the recommendations of the
international scientific and conservation community, have not been adopted by
the plenary session of ICCAT.
Those that have been adopted are, in some cases, not scientifically
supportable or implemented and enforced. Existing data on the biology of the
species is not utilized in the existing population models and fundamental
assumptions in the models are routinely violated and not supported by current
research.
The future of the
Mediterranean bluefin tuna resides in a legitimate commitment to action policies
by the political leadership of all countries that use the resource as producers
and consumers, or both.
Any hope for a recovery to population levels of the past will require the
following four actions:
1)
New
regulations, such as a minimum size of 30 kg, or amendments to existing ones,
must be based upon excellent objective science.
2)
A
significant reduction in total landings and modification of fishing practices of
all countries that harvest bluefin tuna.
3)
An
elimination of illegal, underreporting and unregulated boats and international
censure of nations which tolerate them.
4)
Complete,
accurate and timely reporting of all countries landing bluefin tuna.
5)
Fair,
equitable and sustained enforcement of existing regulations.
6)
A
full commitment by all fishing nations, whether members of ICCAT or not, to a
bluefin rebuilding program.
It is imperative that we all
adopt a new attitude and respect for the fishery resources of the sea.
Based upon past history and actions, one cannot be optimistic that there
will be a positive outcome for this great fish, the tunny of Aeschylus,
Aristophanes and Aristotle, unless political leadership takes courageous and
dramatic action.
Acknowledgements
This project (4-A-30) was
supported by the Italian Ministry for Agriculture, Policy and Forestry,
Department of Fisheries and Aquaculture and the Belle W. Baruch Institute for
Marine Biology and Coastal Research, University of South Carolina, Columbia, SC.
We wish to thank the many students from the Department of Animal Biology
and Ecology of the University of Cagliari that provided so much technical
assistance. This
project would not have been possible without them.
This is publication number XXXX of the Belle W. Baruch Institute for
Marine Biology and Coastal Research of the University of South Carolina.
References
Addis, P., A. Cau, M.A. Davini,
E. Secci, G. Scibaldi. 1996.
Collection of Tuna Data catches by Trap-Nets in Sardinia: Historical (1825-
1980) and Recent Catches (1992-1999). ICCAT.
Coll. Vol. of Sci. Papers, Vol. XLVII(2)132-138.
Bok, T. 1991. Beykoz
dalyani’nin isleyisi ve avciligi uzerine arastirmala.
I.U. Fen Bilimleri Enstitusu, Yuksek Lisans Tezi.
Istanbul-Turkiye
Cau, A.,
Addis P., Cuccu D., Davini M.A., Follesa M.C., Murenu M., Sabatini A., Secci E.,
Dean J.M. 2000. Final Report for the Italian Ministry for
Agriculture, Policy, and Forestry: Methodology for increasing the knowledge of
large pelagic fish assessments of Sardinian Seas.105 ppg. Department of Animal
Biology and Ecology, University of Cagliari, Cagliari, Sardinia, Italy.
Cetti,
F. 1777, Storia naturale della Sardegna. Anfibi e Pesci di Sardegna. Tomo III
Sassari.
Clay, D.,
and T. Hurlbut. 1985. Catch-at-age and estimates of growth of Canadian bluefin
tuna. ICCAT. Coll.Vol. of Sci. Papers. (SCRS/85/24). Vol. XXIV).
143-148.
Cort, J.L. 1990. Biologia y
pesca del atun rojo (Thunnus thynnus)
del mar Cantabrico. Publicaciones Especiales Inst. Esp.
Oceanog., 4 :272 pp.
de la
Serna, J.M., E. A lot, M.P.
Rioja. 1997. Proporcion de sexos y sex-ratio por clase de talla del atùn
rojo (Thunnus thynnus) capturado por las almadrabas atlàntica espanolas
durante el periodo 1989-1995. ICCAT, Coll.
Vol. of Sci. Papers; Vol. XLVI (2); 97-101.
Farrugio,
H. 1980. Bluefin fishing in the French area of the Mediterranean.
Development and characteristics. ICCAT, Coll. Vol. of Sci. Papers; 11:
98-117.
Food
and Agriculture Organization. 1995a: Precautionary Approach to Fisheries. FAO
Fisheries Technical Report 350. United Nations, Rome.
Forberg
K. G. 1982. A histological study of development of oocytes in capelin, Mallotus
villosus (Muller). Journal of fish Biology, 20: 143-154.
Fromentin,
J, A. Fonteneau, H. Farrugio. 2000. Biological reference points and natural long-term
fluctuations: the case of the eastern Atlantic bluefin tuna. ICCAT, Coll. Vol. of Sci. Papers. SCRS/99/54
11:98-117
Hutchings,
J.A. 2000. Numerical assessment in the front seat, ecology and evolution in the
back seat: time to change drivers in fisheries and aquatic sciences? Mar. Ecol. Prog. Ser., 208:
299-303.
ICCAT. 1975. Report for the biennial period. Part I
(1974). Madrid, Spain.
ICCAT.
1997a. Coll. Vol. of Sci. Pap.- Vol. XLVI (1). Table 4, 122-123pp, Table 15,
161pp. Madrid, Spain
ICCAT.
1997b. Coll. Vol. of Sci. Pap. Vol. XLVI (2) Bluefin Tuna (T.thynnus) - GFCM/ICCAT
Session. Madrid, Spain.
ICCAT,
1999. Annex 5-5, Recommendations & Resolutions Adopted by the Commission.
Report for the biennial period. Part I (1998) - Vol 1. ICCAT, Madrid, Spain.
ICCAT,
2000a. Statistical Bulletin, Vol. 30 1999, ICCAT, Madrid, Spain.
ICCAT,
2000b. Report for biennial period, 1998-1999, Part II (1999)-Vol. 2. ICCAT,
Madrid, Spain.
ICCAT,
2001. Executive Summary, 1-4, Proceedings of the 12th Special Meeting
of the Commission. ICCAT, Madrid, Spain.
ICCAT,
2001. Report for biennial period, 2000-2001, Part I (2000)-Vol. 2. ICCAT,
Madrid, Spain.
Kalish, J., J.M. Johnston, and
T. Matsumoto. 1998. Age of bluefin tuna estimated from radio carbon dating using
the bomb radiocarbon chronometer to determine age of Atlantic bluefin tuna (Thunnus
thynnus). ICCAT, Coll. Vol. of Sci. Papers. Vol. XLVIII (1). SCRS/98/81.
Yamamoto
K. 1956. Studies on the formation of fish eggs. I. Annual cycle in the
development of ovarian eggs in the flounder L.
obscura. Journal
of the Faculty of Science Hokkaido University, Series VI, Zoology (12):
362-373.
Manconi,
F., Le miniere e i minatori della Sardegna. 1986. Silana Editoriale, 237 pp.
Myers,
R.A. and G. Mertz. 1998. The limits of exploitation: a precautionary approach.
Ecol. Appl. 8(1) Supplement. S165-S169.
Nelson,
K. and M. Soulé. 1986. Genetical conservation of exploited fishes, 345-368. In
Population genetics and fishery management. Ed. by N. Ryman, and F. Utter.
Washingon Sea Grant Program, Unif. Of Washington Press, Seattle, USA.
Nishida,S. Tsuji, K. Segawa.,1998.
Spatial data analyses of Atlantic bluefin tuna larval surveys in the 1994 ICCAT
BYP. ICCAT,
Coll. Vol. Sci. Papers. Vol. XLVIII (1). 107-110.
Olla, R.
2000. Biologia riproduttiva di Thunnus
thynnus ( L.): fecondità e sviluppo dell’ovario. Tesi di Laurea,
Università di Cagliari, Italy.
Oray,
I.K. and S. Karakulak, 1997. Some remarks of the bluefin tuna (Thunnus thynnus ( L.) 1758, fishery in Turkish waters in 1993, 1994,
1995. ICCAT Symp. 96/030. 357-362 XLVI (2).
Oray,
I and S. Karakulak. 1999. Investigations on the reproductive biology of bluefin tuna (tuna (Thunnus
thynnus.L. 1758) in the North Aegean Sea. ICCAT, Coll. Vol. Sci. Papers.
Vol. XLIX (1). SCRS/98/56.
Piccinetti, C.1980. The
bluefin seine fisheries in the Adriatic. ICCAT. Coll. Vol. of Sci. Papers, 11: 346-350.
Piccinetti, C.,
Picccinetti-Manfrin, G., S., Soro. 1997. Résultats d’une campagne de recherche sur les larves de thonidés en Méditerranée.
ICCAT,
Coll. Vol.of Sci. Papers.
(Vol. XLVI (2) );207-214.
Purdom, C.E. 1979. Genetics of
growth and reproduction in Teleosts. Symp. Zool.Soc. London. 44:207-217.
Rey,
J.C. and J.L. Cort. 1986. The tagging of the bluefin tuna (Thunnus
thynnus) in the Mediterranean: history and analysis. CIESM, 2pp.
Rijnsdorp,
A.D., 1993. Fisheries as a large-scale experiment on life-history evolution:
disentangling phenotypic and genetic effects in changes in maturation and
reproduction of North Sea plaice, Pleuronectes
platessa L. Oecologia 96: 391-401.
Rodriguez-Roda,
J. 1964. Biologia del atùn, Thunnus thynnus
(L.) de la costa sudatlàntica Espanola. Inv. Pesq., 25: 33-146.
Roff,
D.A. 1983. An allocation model of growth and reproduction in fish. Can. J. Fish.
Aquat. Sci. 40: 1395-1404.
Rubino,
S. 1996. La Tonnara Saline. La Celere Editrice, Alghero. 288pp.
Smith,
D.M. 1968. A history of Sicily: Medieval Sicily 800-1713. Chatto and Windus,
London, England.
Smith,
J.A. and W.D. Ross. 1910.The works of Aristotle translated into English: the
revised Oxford translation. Volume IV Historia Animalium by D’Arcy Wentworth
Thompson.,Bekker (543 b 35). Oxford at the Clarendon Press.
Srour, A.
1993. Realation
taille-poids et composition en taillesdes captures du thon rouge (Thunnus thynnus) de la Mediterranee marocaine, ICCAT, Coll. Vol.of
Sci. Papers.
(Vol. XL )(1);155-156.
Stearns,
S.C. and R.E. Crandall. 1984. Plasticity for age and size at sexual maturity; a
life history response to unavoidable stress. In: Fish reproduction; strategies
and tactics, 13-33. Ed. by G.W. Potts and R.J. Wootton (eds) Academic Press,
London.
Tyndale,
J.W. 1849. The Island
of Sardinia Vol. III. Pg. 143-179. R. Bentley, London, England.
Tsuji,
S., Y. Nishikawa, K. Segawa, Y. Hiroe., 1997. Distribution and abundance of
Thunnus larvae and their relation to the oceanographic condition in the Gulf of
Mexico and the Mediterranean Sea during May through August of 1994. ICCAT.
Coll.Vol. of Sci. Papers (Vol.XLVI (2);161-176.
Ueyanagi,
Sl, Y. Nishikawa, S. Tsuji. 1997. Identification and occurrence of Thunnus
larvae collected from the Gulf of Mexico and the Mediterranean Sea by the
Shoyo-maru cruise in 1994 with the review of recent identification study of
larval Thunnus. ICCAT. Coll. Vol.of
Sci. Papers. (Vol.
XLVI (2):181-185.
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